1. Field of the Invention
The present invention generally relates to radio transmitters and more particularly to emergency vehicle radio transmission systems.
2. Description of the Prior Art
The effectiveness of acoustical warning systems employed by emergency vehicles, trains, or the like has become increasingly problematic in recent years. In an ideal world, an emergency vehicle's acoustical warning signal, such as a siren, can be heard by nearby motorists and the motorists may respond accordingly. However, for a variety of reasons, the operators of modern vehicles may not be able to hear an acoustical warning provided by a nearby emergency vehicle or the like. First, modern automobiles are more sound proof thereby reducing the sound of an acoustical warning therein. Second, with the widespread use of vehicle air conditioning, more motorists have their windows closed thereby reducing the effectiveness of an acoustical warning system. Third, many motorists may have high performance radios installed in their vehicles, whereby the high performance radios may be operated at a relatively high volume level thereby drowning out the acoustical warning provided by a nearby emergency vehicle or the like. Any one, or a combination of the above mentioned factors, has reduced the effectiveness of acoustical warning systems.
There have been several advancements in recent years to help alleviate this problem. U.S. Pat. No. 5,278,553, issued to Cornett et al., suggests a system wherein a microphone and a sensing system are placed in each motor vehicle. The microphone is placed near the front or rear windshield and provides an electrical signal to a control block. The control block filters the electrical signal to detect a sounding siren. Once a sounding siren is detected, the control block over-rides the radio receiver in the corresponding motor vehicle and provides a warble sound to the driver, indicating an approaching emergency vehicle. The system suggested by Cornett et al. may be expensive to implement because each motor vehicle must be provided with a microphone and a control block as described above. With millions of motor vehicles on the road, this could result in hundreds of millions of dollars in implementation costs.
Similar approaches are suggested in U.S. Pat. No. 3,873,963, issued to Neal et al., and U.S. Pat. No. 3,859,623, issued to Koehler.
U.S. Pat. No. 4,403,208, issued to Hodgson et al., U.S. Pat. No. 4,241,326, issued to Odom, U.S. Pat. No. 4,238,778, issued to Ohsumi, U.S. Pat. No. 3,760,349, issued to Keister et al., U.S. Pat. No. 3,673,560, issued to Barsh et al., and U.S. Pat. No. 3,710,313, issued to Kimball et al., all require additional equipment to be installed in each motor vehicle wherein the additional equipment receive signals from an emergency vehicle and provide a warning to a corresponding driver. These systems suffer from the same limitations as discussed above.
Another advancement is suggested in U.S. Pat. No. 4,443,790, issued to Bishop. In Bishop, an emergency vehicle provides a siren signal over all AM and FM band frequencies such that any radio receiver in the region, tuned to any AM or FM frequency, may receive said siren signal and provide a warning to a corresponding driver. In this approach, no additional equipment need be installed in a corresponding motor vehicle. However, Bishop suffers from a number of limitations. A first limitation is that significant power may be required to transmit over all frequencies in the AM and FM band. A second limitation is that Bishop only contemplates sweeping all AM or FM bands at a rate of 150 to 450 Hz, which severely limits the quality of the audio signal that can be provided to any given radio receiver. That is, Bishop may only provide a warning tone or equivalent to a corresponding driver.
Another system related to Bishop is described in U.S. Pat. No. 4,764,978, issued to Argo et al. Like Bishop, Argo suggests broadcasting a siren signal on each and every AM and FM frequency. However, Argo suggests broadcasting on each and every AM and FM band simultaneously. A limitation of Argo is that a significant amount of power and hardware may be required to simultaneously transmit over all frequencies in the AM and FM band.